Geophysicist studies the loud crackling noise of glaciers melting

The crackling, popping sounds of ice melting in underwater
glacial fjords -- caused by trapped air bubbles escaping -- are
responsible for the loudest natural marine environments on
Earth.

Geophysicist Erin Pettit
from the University of Alaska has studied and recorded the
phenomenon using underwater microphones in order to help us better
monitor fast-changing polar environments.

Pettit had often heard the sounds while out kayaking, before she
set up a network of underwater hydrophones off the Alaskan coast.
These are suspended around 100 metres from the ocean floor.
Underwater, the sound was much louder.

"If you were underneath the water in a complete downpour, with
the rain pounding the water, that's one of the loudest natural
ocean sounds out there," she explained in a press release. "In
glacial fjords we record that level of sound almost
continually."

Suspecting the din was caused by the ice melting, Pettit devised
a more controlled experiment, teaming up with acoustics experts
Kevin Lee and Preston Wilson from the University of Texas. She sent the
pair chunks of the glacier, which were placed into a tank of
chilled water. They then recorded video and audio of the melting
process and could match the sound to the sight of the bubbles
escaping.

Lee and Wilson found that most of the sound came from bubbles
oscillating as they are ejected from the ice. "A bubble when it is
released from a nozzle or any orifice will naturally oscillate at a
frequency that's inversely proportional to the radius of the
bubble," Lee said.

The bubbles get formed because layers of snow crystals trap
little pockets of air. As more snow falls on top of those layers,
the snow becomes compacted into ice, trapping pressurised bubbles
of air.

The researchers believe that they could use sound recordings
taken in glacial fjords to monitor the melting of ice to complement
time-lapse photography.